The present invention relates to a powder-metallurgical extrusion process for the production of cylindrical metallic or ceramic blanks having one or more internal twisted longitudinal bores and an extrusion tool therefor.
An extrusion process and extrusion tool for producing a hard metal or ceramic drill blank having at least one twisted internal coolant bore are described in EP-A1 0-223 909. The coolant bores in the drill blank are formed by central pins fastened to a mandrel. The pins are straight and are produced from a flexible material such as copper or a copper alloy. In this process, the drill blank is turned by helical webs or grooves on the die's inner surface during extrusion through the die. As a result, twisted internal coolant bores are formed inside the drill blank, but spiral webs or grooves are also formed on the outer surface of the drill blank due to the helical webs or grooves on the die. Hence, the outer surface of the blank requires grinding to smooth the surface.
Another disadvantage of the above process is that, due to the flexibility of the pins, precise formation of the coolant bores within the drill blank is not possible. Also the position of the coolant bores within the individual blanks can vary greatly. Consequently, this may cause uneven wall thicknesses between the coolant bores and the blank's outer surface, sometimes resulting in the coolant bore being completely exposed during grinding of the outer surface, thus causing the blank to be rejected.
The object of the present invention is to provide a process for the production of a cylindrical blank having one or more internal twisted longitudinal bores, in which the position of the twisted longitudinal bores is as precise as possible within the cylindrical blank and uniform or nearly uniform for each blank produced.